I. Field of the Invention
This invention relates generally to a method and apparatus for dehydrating "green" bio-materials, such as freshly cut wood chips, and more specifically to an improved baling press whereby an increased water movement from the center of the biomass to the edges thereof is achieved.
II. Discussion of the Prior Art
In preparing biomass material, such as wood chips, for later use as an industrial fuel, it is generally desirable that the moisture content of the fuel be reduced as much as is practical, given the amount of energy which is expended in carrying out the drying process.
It is known in the art that wood chips may be mechanically dewatered by passing the "green" wood chips through roll or nip presses. Such presses are less than satisfactory from an efficiency standpoint in that they are able to reduce the moisture content of wood chips and bark residues to only about 100 percent on a dry-weight basis. By definition, a 100 percent moisture content means that the weight of the water in the wood chips is equal to the weight of the dry wood itself. In that many woody biomass materials harvested for use as fuels are only slightly wetter than this 100 percent moisture content (MC) immediately upon harvesting, the use of roll or nip presses cannot be economically justified.
In papers which I published in the Forest Products Journal (Volume 31, No. 8 and Volume 32, No. 10) it was reported that by using a batch-type baling press it appeared feasible to reduce the moisture content of green wood chips to about 55 percent. That estimate, which was based upon experimental work involving a laboratory-scale baling press, incorporated an enclosed chamber having a laterally-movable bottom member and a hydraulically-operated ram for squeezing a charge of green wood chips against that movable bottom member, utilizing a flat ram face which results in a practically zero pressure gradient being built up across the pressure chamber as the ram is folded downward on the biomass charge.
The Strickland U.S. Pat. No. 4,036,359 describes a baling press arrangement for de-watering wood chips, but it, too, describes a ram face configuration which is flat or planar. The application of high pressure alone is not sufficient to ensure effective compression drying of biomass materials. No matter what type of mechanical system is employed, it is important that a means be provided for removing the expelled water from contact with the mat of compressed biomaterials prior to the release of the pressure. Otherwise, upon release of the pressure, the compressed biomass materials would tend to expand and, if the expelled water is still in contact with the biomass, it would be drawn into the material.
The necessity of removing the water from contact with the biomass while it is still under pressure led me to conclude that certain mechanisms for applying pressure may be more effective than others.
I have theorized that if a substantial pressure gradient can be established within the compressed biomass mat immediately upon closing the press, an increase in the rate of water removal as well as the extent of removal can be enhanced. To my knowledge, while several attempts have been made to de-water wood chips in a baling press, each case a flat ram face has been used. No one, to my knowledge, has attempted to improve the efficiency of the de-watering process through modification of the shape of the ram face in a fashion to increase the establishment of a desired pressure gradient.